Batteries to provide small current for electronic products are known in the art. For instance, R.O.C. Patent Publication No. TW568363, entitled “Battery to provide small current for electronic products” and U.S. Patent Publication No. 20040125527, entitled “Battery for an electrical appliance that consumes low voltage battery” are such examples. They have two electrodes in contact with active carbon to generate chemical reactions with water to provide electricity. However, no isolation membrane is in between the two electrodes and their structure is complex and difficult to be miniaturized. Short circuit tends to occur between the two electrodes, and the shrunk water container cannot hold a desired amount of water.